1. Field of the Invention
The present invention relates to a projection optical system and an exposure apparatus.
2. Description of the Related Art
It is effective for an i-line projection exposure apparatus to intensify the light energy and to secure a broader exposure area so as to improve the throughput. However, the light absorbed in a glass material is turned into heat, and thus an improvement of the durability of the projection optical system to the light energy is necessary in intensifying the light energy.
In order to restrain the influence of the light energy, Japanese Patent Laid-Open No. (“JP”) 2003-202494 discloses a projection optical system that includes a negative lens closest to an image plane made of a glass material having a high transmittance such that the lens can have a refractive index of 1.55 or smaller and an appropriate power. JP 2004-046119 discloses a projection optical system that restrains the influence of the light energy by limiting use volume of a less durable glass material having a high refractive index and a low Abbe number.
Nevertheless, each of the embodiments in JP 2003-202494 uses a less durable glass material having a refractive index of 1.57 or higher for the entire projection optical system, and the overall projection system is too thick to sufficiently reduce the influence of the light energy, causing its imaging characteristic to deteriorate. In addition, in each of the embodiments of JP 2004-046119, maximum heights of the object plane (first object plane) and the image plane (second object plane) are too small to secure a broad exposure area. Thus, the conventional i-line projection exposure apparatus cannot sufficiently improve the throughput.